This invention relates to a strain sensor formed on a surface of an elastic body for detecting strain produced due to bending of the elastic body and, in particular, to a capacitive strain sensor for detecting the produced strain as a change in capacitance, to a method of using the same for detecting an internal pressure of a hollow-cylindrical closed container made of the above-mentioned elastic body with reference to expansion/depression in its upper flat end portion, for detecting cylinder torsion produced on an outer peripheral surface of a bar-like cylindrical elastic body, and for detecting an acceleration, and to a method of detecting the strain and correcting the same.
As a strain sensor formed on a surface of an elastic body for detecting strain produced due to bending of the elastic body, there has been well known a so-called strain gauge having an electric resistance changing in value in response to the strain.
FIG. 1 is a perspective view showing an example of the state of use of a conventional strain sensor. FIG. 2 is a perspective view showing an example of a strain gauge used as the conventional strain sensor.
In FIG. 1, a cylindrical elastic body 20 is placed on one surface of a mount table 10 with a strain gauge 30A adhered to a center area of an upper end surface 21 of the elastic body 20 and a strain gauge 30B adhered to an outer peripheral side surface 22 so that its strain detection axis is inclined by 45xc2x0 with respect to a center axis direction of the elastic body 20 to coincide with a strain direction.
The strain gauge 30 shown in FIG. 2 is one identical with the strain gauges 30A and 30B shown in FIG. 1 and comprises a resistance wire 31 having a parallel-line thin film pattern made of an Fexe2x80x94Ni alloy and formed by making a plurality of turns, and terminals 32 and 33 arranged at opposite ends thereof.
At first, the strain sensor 30A shown in FIG. 1 will be described in connection with the case where the cylindrical elastic body 20 is a closed container and the strain sensor is used in detecting an internal pressure of the closed container elastic body 20.
When the internal pressure of the closed container elastic body 20 is increased from a normal condition, the closed container elastic body 20 is deformed and expands in the upper end surface 21, the outer peripheral side surface 22 and a lower end surface thereof. When the internal pressure of the closed container elastic body 20 is decreased from the normal condition, the elastic body 20 is deformed and is depressed in the upper end surface 21, the peripheral side surface 22 and the lower end surface thereof. Accordingly, if the wall thickness of the upper end 21 with the strain gauge 30A adhered thereonto is slightly reduced within a safety range as compared with those of the peripheral side and the bottom end, the variation in internal pressure can be converged to the deformation of the upper end 21.
The strain gauge 30 shown in FIG. 2 is a sensor in which a resistance value of a conductor formed in the thin film pattern changes under the strain applied thereto. When the internal pressure of the closed container elastic body 20 shown in FIG. 1 is varied, the upper end 21 is at first deformed so that the strain gauge 30A adhered thereto is similarly deformed. This results in variation in electrical resistance between the both terminals 32 and 33 of the resistance wire 31 of the strain gauge 30. Thus, it is possible to detect the internal pressure of the closed container made of the elastic body 20.
Next, the strain sensor 30B shown in FIG. 1 will be described in connection with the case where the elastic body 20 having a cylindrical shape, that is, the cylindrical elastic body 20 is a cylinder having a circular-shaped cross section in the outer peripheral surface and the strain sensor is used in detecting torsion of the cylinder.
In the above-mentioned state, it is assumed that the torsional strain of the cylindrical elastic body 20 is detected by the use of the strain gauge 30B. When a torsional moment is applied to the cylindrical elastic body 20 to produce the torsional strain in the cylindrical elastic body 20, an extension strain in a direction inclined by 45xc2x0 with respect to the center axis direction of the cylindrical elastic body 20 and a compressive strain in a direction perpendicular thereto are produced at the portion where the strain gauge 30 is adhered. Therefore, it is possible to detect the torsional strain of the cylindrical elastic body 20 by detecting the change in resistance value depending on the extension strain and the compressive strain.
On the other hand, the strain sensor can be used as an acceleration sensor. The acceleration sensor is used in detecting the vibration of a car and the acceleration upon collision thereof, the vibration and the acceleration applied to an electronic apparatus when it is carried, and the abnormal vibration of a motor and various kinds of machines. In order to detect the vibration and the impact of those machines, many kinds of acceleration sensors have been used. Depending on the magnitude and a frequency range of the acceleration to be detected, use has been made of a selected one of the acceleration sensors which is suitable for the application.
Next, referring to FIGS. 3 to 5, description will be made about a conventional acceleration sensor which is used, for example, in detecting the vibration caused by knocking of a car engine or the vibration of a machine.
An acceleration sensor 40 shown in FIG. 3 has a structure in which two piezoelectric rings 41 and 42 are stacked so that their polarization directions are opposite to each other and are fixed by a fixing screw 44 together with a weight 43 comprising a hollow metal cylinder, and is used in detecting the acceleration of the order of several Gal to several tens of Gal. The acceleration sensor 40 is provided with a case 45 generally connected to the ground, specifically, connected via a mounting screw 47 to a ground terminal of an object to be detected, together with a terminal 46 similarly grounded.
In the acceleration sensor 40 of FIG. 3, when an acceleration xcex14 is applied from the outside, the piezoelectric rings 41 and 42 are subjected to the force xe2x80x9cF4=M4xcex14xe2x80x9d. Herein, M4 represents the mass of the weight 43. Each of the piezoelectric rings 41 and 42 is an element which produces an electric voltage under a pressure applied thereto, as is self-explanatory, and produces the electric voltage given by xe2x80x9cV=kxc2x7gxc2x7F4xe2x80x9d. Herein, xe2x80x9ckxe2x80x9d and xe2x80x9cgxe2x80x9d represent a constant determined by the shape and the size of the acceleration sensor and another constant determined by a piezoelectric material, respectively. Thus, the principle of an operation of a piezoelectric-type acceleration sensor represented by the acceleration sensor 40 shown in FIG. 3 is that an applied acceleration acts on the weight 43 to produce a force and the piezoelectric rings 41 and 42 are deformed under the force to produce the electric voltage.
Recently, development has been made of a capacitive acceleration sensor of a so-called micromachine type produced by making the most of a semiconductor micromachining technique. This sensor can detect the acceleration on a d.c. basis and can accommodate a wide range from a small acceleration not greater than 1 Gal to a large acceleration of several tens of Gal upon collision of the car by designing a resonant frequency of a mechanical vibration system and a mechanical strength of each part to meet such a requirement.
FIG. 4 is a schematic perspective view showing an example of a structure of a capacitive acceleration sensor 50 using the micromachining technique. The capacitive acceleration sensor 50 comprises an Si single crystal plate 51 which is formed, by a surface micromachining technique, with movable electrodes 55(X) integral with a movable plate 54 which serves as a weight and which is supported by supporting portions 53 fixed to anchors 52, and two sets of fixed electrodes 56(Y) and 57(Z) faced to the movable electrodes 55(X) to establish the capacitances.
Next, referring to FIG. 5 in addition to FIG. 4, description will be made about the principle of detecting the acceleration. FIG. 5 is a view for describing an operation of the capacitive acceleration sensor 50 shown in FIG. 4.
In FIG. 5, X(55) electrodes, Y(56) electrodes, and Z(57) electrodes are connected in common, respectively, which can resultantly be considered as a circuit comprising two capacitors connected in series where the electrode X(55) is disposed between the electrodes Y(56) and Z(57) faced to each other as illustrated in FIG. 5. In FIG. 4, the direction of the detected acceleration is a direction perpendicular to a longitudinal direction of each of the electrodes X, Y, and Z and parallel to a plane of the Si single crystal plate 51. Accordingly, when an acceleration xcex15 is applied, the force xe2x80x9cF5=M5xcex15xe2x80x9d is produced, like in the case of the piezoelectric-type acceleration sensor. In this case, the mass M5 represents the mass of the movable plate 54 including the movable electrodes 55(X). When the force F5 is produced, the movable electrodes 55(X) are displaced to positions where the balance is established with elastic force of the supporting portions 53 fixed to the anchors 52. Specifically, in FIG. 4, the movable electrodes 55(X) are shifted from the center position towards the fixed electrode 56(Y) or 57(Z).
In FIG. 4, it is assumed that the fixed electrodes 56(Y) and the fixed electrodes 57(Z) are applied with electric voltages Vy and Vz different in phase by 180xc2x0 from each other and equal in amplitude, respectively. In this event, when the movable electrodes X(55) are positioned at the center between the fixed electrode 56(Y) and the fixed electrode 57(Z), an electric voltage Vx of the movable electrode X(55) is equal to zero because of mutual cancellation. On the other hand, when the acceleration is applied so that the movable electrode X(55) is shifted from the center between the fixed electrode Y(56) and the fixed electrode Z(57), the electric voltage Vx is produced at the movable electrode X(55). The level of the electric voltage is proportional to the displacement of the movable electrode X(55), that is, the magnitude of the applied acceleration. Accordingly, it is possible to detect the applied acceleration from the electric voltage at the movable electrode X(55).
However, in the above-mentioned strain gauge, a metal thin film having resistance changing in response to the strain applied thereto is formed on a thin substrate, such as polyimide, by means of vapor deposition or the like. In actual detection of strain, the thin substrate such as polymide must be adhered by the use of an adhesive to a plate to be subjected to detection of the strain. Since a detection characteristic will vary depending upon an adhering position or the fluctuation in thickness of an adhesive layer, the characteristic must be confirmed upon each adhesion.
As another means for detecting the deformation of a flat plate, there is a method where, providing that the flat plate is a metallic plate which is subjected to a deformation to be detected, another metal plate is disposed to face the above-mentioned metallic plate to detect, as the change in capacitance between two electrodes, the change in spacing between the metal plates facing each other in response to the deformation of the metallic plate to be detected. In this method, however, it is necessary to accurately hold the positional relationship between the metallic plate to be detected and the other metal plate separately arranged. Therefore, the structure is complicated, resulting in difficulty in manufacturing.
In case of a torsion sensor on a cylindrical side surface, it is further necessary to adjust an inclination angle with respect to an axial direction. In principle, the resistance value varies depending on the change in tensile strain and compressive strain. Therefore, if signal processing is carried out by the use of a microcomputer or the like, an analog-to-digital conversion circuit is required. As a result, a signal processing circuit is complicated.
The strain gauge using a semiconductor is advantageous as the strain sensor because the sensitivity is as high as several ten times because of a piezo resistive effect, as compared with the strain gauge using the metal. However, the strain gauge using the semiconductor is disadvantageous in that the temperature-dependent change in resistance is large and the strain sensitivity varies with the strain level. Accordingly, if a small strain is measured, the structure becomes complicated.
The piezoelectric type acceleration sensor described with reference to FIG. 3 is advantageous in that its structure is simple and no power supply is necessary in principle. However, in the state where an acceleration on a d.c. basis, that is, a constant force is applied to a piezoelectric element, electric charges produced by the deformation leak out through an electronic circuit for detection and a surface or an interior of a piezoelectric material so that the electric voltage is reduced. Consequently, it is difficult to correctly detect the applied acceleration.
In the capacitive acceleration sensor described with reference to FIGS. 4 and 5, the movable electrode and the fixed electrode are required in order to establish the capacitance. The applied acceleration is at first converted into the change in spacing between the movable electrode and the fixed electrode and, as a result, converted into the change in capacitance. Accordingly, in order to accurately form the movable electrode and the fixed electrode, an expensive equipment capable of achieving a high working accuracy is required. Thus, this acceleration sensor is unfavorable.
Therefore, it is an object of this invention to provide a strain sensor which is simple in structure and stable in characteristic, which can accurately detect very small strain, and which can carry out correction not only for the temperature but also for other environmental factor such as the humidity. It is a further object to provide a strain sensor which has an advantage of a capacitive acceleration sensor capable of detecting an acceleration on a d.c. basis and which can be used as an acceleration sensor capable of detecting the change in capacitance responsive to deformation of an elastic body when the acceleration is applied, without using two electrodes, that is, a movable electrode and a fixed electrode, but using a single element.
A capacitive strain sensor according to this invention comprises a substrate and at least one interdigital pair-electrode capacitor formed thereon. The substrate is an elastic member having a flat or a curved surface on which a dielectric film layer is formed with a substantially uniform thickness and made of a material having a dielectric constant changing in dependence upon the strain. The interdigital pair-electrode capacitor comprises at least a pair of electrodes which are formed on a surface of the substrate as parallel linear electrodes comprising a plurality of linear conductors and which are combined in an interdigital pattern.
In a method of using the above-mentioned capacitive strain sensor, provision is made of oscillation means for generating frequency modulation depending on the change in capacitance of the capacitive strain sensor so that a strain level is detected from fluctuation in frequency of an oscillation signal produced by the oscillation means. Specifically, the capacitive strain sensor is incorporated as a capacitor element in the oscillator circuit so that the magnitude of the strain responsive to the deformation of the elastic body can be converted into the change in capacitance or impedance and, furthermore, can readily be converted into the change in frequency. Since a very small change can be extracted in the frequency after conversion, it is possible to easily and reliably detect even a slight change in strain.
In the above-mentioned capacitive strain sensor, use of a single interdigital pair-electrode capacitor limits a direction to be able to be detected thereby. By the use of two interdigital pair-electrode capacitors arranged so that the directions of digits in the interdigital patterns formed by the linear electrodes are substantially perpendicular to each other, it is possible to remove the limitation in directivity of the strain sensor itself. Furthermore, the capacitive strain sensor may have a structure in which a plate-like elastic body is a flat plate having a disk shape or a polygonal shape including a square and the interdigital pair-electrode capacitor is formed so that the linear electrodes as the digits of the fingers of the interdigital pattern are arranged in substantially concentric circles around the center of the flat plate. With this structure, radial strain can reliably be detected.
If the interdigital pair-electrode capacitors are formed on upper and lower ends of a cylindrical closed container comprising the elastic body, the capacitive strain sensor is effectively used in detecting the change in internal pressure of the closed container by detecting the strain of the ends as the change in capacitance of the capacitors.
The capacitive strain sensor according to this invention may be one wherein the elastic body is formed in a rectangular structure and the interdigital pair-electrode capacitor comprises linear conductors and has a capacitance by forming a pair of electrode patterns including a pair of common electrodes extending in parallel to two parallel sides of the rectangular elastic body and faced to each other and a plurality of linear electrodes inclined by about 45xc2x0 with respect to an extending direction of the common electrodes and extending in parallel to one another to be alternately interposed as the digits of the interdigital pattern. The above-mentioned electrode patterns are suitable for a cylindrical structure in which the elastic body extends in one axis direction as a center axis and has an outer peripheral surface having a circular section. Specifically, the dielectric film layer is formed on the outer peripheral surface of the cylindrical structure to be substantially uniform in thickness. The interdigital pair-electrode capacitor has a capacitance by forming a pair of electrode patterns including a pair of common electrodes forming ring patterns around an outer peripheral surface of the cylindrical structure and faced to each other and a plurality of linear electrodes inclined by about 45xc2x0 with respect to the center axis direction and spirally extending from the common electrodes in parallel to one another to be alternately interposed as the digits of the interdigital pattern.
If two equal parts divided in a center axis direction or in a direction parallel to the center axis are formed on an outer peripheral side surface of the cylindrical structure comprising the elastic body and the directions of the linear electrodes as the digits in two groups are perpendicular to each other, the above-mentioned capacitive strain sensor is effective in detecting the strain for torsion in both directions.
The capacitive strain sensor according to this invention may be one wherein the elastic body has a flat plate shape and is provided with a fixed portion formed on one end thereof and having a structure for preventing production of the strain and that a single interdigital pair-electrode capacitor is formed on the surface of the elastic body except the fixed portion. Preferably, the capacitive strain sensor further comprises a weight formed on the surface of the elastic body except the fixed portion of the elastic body to promote the bending of the elastic body. By adding the weight, it is possible to effectively detect an applied acceleration.
The above-mentioned capacitive strain sensor has a simple structure and can form an LC oscillator circuit or an RC oscillator circuit. The change in output frequency of the oscillator circuit depending on the change in capacitance is very effective in accurately and exactly detecting a small strain level by voltage conversion.
However, since the capacitance of the capacitive strain sensor varies not only following the change in strain level but also following the change in ambient temperature, a correction circuit is required to correct the change in characteristic of the capacitance depending upon the temperature.
For this purpose, in the capacitive strain sensor according to this invention, at least one of an interdigital pair-electrode capacitor arranged as a reference or a separately arranged capacitor is provided as a reference capacitor. The capacitance of the reference capacitor is used for correction in strain detection. The elastic body with one interdigital pair-electrode capacitor formed thereon is provided with a fixed portion formed on one end thereof and having a thickness such that the strain is hardly produced or having a fixed structure for preventing production of the strain. The reference capacitor including the reference interdigital pair-electrode capacitor is formed on the fixed portion. Accordingly, even if the strain is produced in the elastic body, the reference capacitance is unchanged. Of course, the reference capacitor may be formed on a fixed body separate from the elastic body.
By comparing the capacitance or impedance of the above-mentioned reference capacitor with that of the interdigital pair-electrode capacitor for detecting the strain and by carrying out correction, it is possible to eliminate external conditions affecting the interdigital pair-electrode capacitor except the strain.